1. Field of the Invention
The present invention relates generally to the manufacturing process of semiconductor devices, and more particularly to forming aluminum bond pad regions to avoid corrosion.
2. Background
Microelectronic circuit manufacturers typically manufacture integrated circuits with interconnect bond pad regions exposed in openings of a top passivation layer. The metal pad provides a region for interconnection of components in the integrated circuit to external components. A bonding wire is soldered or otherwise attached to the pad to enable connection to the external circuitry. Vias connect the pad internally to a lower metalization region which forms an interconnect line to connect to components in the integrated circuit.
Corrosion of aluminum pad regions can occur both during manufacturing, and afterward because the aluminum pads are exposed in non-hermetically sealed plastic packages. Corrosion can occur during manufacturing because of fluorine used in passivation etching to expose the pad regions. The fluorine etchant is typically applied to remove unmasked portions of the top silicon layer of a chip to expose the pad regions. The passivation etch typically contains sulfur hexa-fluoride (SF6). The fluorine and sulfur in SF6 causes corrosion of the aluminum pads.
The pad regions of the integrated circuit are not typically hermetically sealed to prevent corrosion. Instead, the pads are left exposed to enable bond wires to be attached. Exposure of the aluminum pad to fluorine during the manufacturing process results in corrosion of the aluminum pad. Other environmental contaminants can also cause undesirable corrosion to the exposed aluminum pads after manufacturing is complete.
Thermosonic ball-bonding is used to attach an interconnect wire or ribbon to the metal bond pad when the integrated circuit contains heat sensitive components. Thermosonic bonding uses a combination of a relatively low temperature, pressure, and a high frequency to bond the ribbon or wire conductor to the metal interconnect pad which provides connectivity to the sensitive circuitry. Relatively low temperature indicates a temperature no greater than the temperature that would potentially cause a modification of the circuit parameters of at least one of the system components. Such a temperature may range up to 150 degrees Celsius, significantly lower than typical soldering temperatures. A substrate is typically heated by the way of a heating plate upon which the integrated substrate is clamped, and pressure is further applied to the substrate.
The temperature is applied while the ultrasonic bonding frequency ranging from 60 KHz up to 140 KHz is applied to the wire bonding lead. The combination of the application of high temperature, pressure, and the moderate ultrasonic frequency abrasion operates to effect metallurgical atomic diffusion bonding of the wire bond with the metal pad bonding site. The high frequency range achieves the requisite atomic diffusion bonding energy, without causing fracturing or destruction of the bonding wire or its interface with the metal bond pad.
It is desirable to improve the reliability of the thermosonic gold to aluminum wire bonds. To provide such reliability, it is desirable to provide a pad structure for an integrated circuit which minimizes corrosion during manufacture and afterward when the integrated circuit is provided in a non-hermetically sealed plastic package.
In accordance with the present invention a method is provided to improve the reliability of gold to aluminum thermosonic bonds by plating the aluminum bond pads to prevent corrosion.
To prevent corrosion, the aluminum bond pads are sealed with gold or silver plating. The gold or silver plating serves to eliminate corrosion due to exposure of the aluminum to fluorine during the manufacturing process. The plating further prevents corrosion of the aluminum pads in the non-hermetically sealed plastic packages after manufacture.
To reduce corrosion prior to plating the aluminum pad, an argon sputter is applied to remove corrosive contaminants remaining from the passivation etch. The argon sputter etch is limited to remove only approximately 100 angstroms of aluminum from the pad regions. Next the 100-200 xc3x85 layer of gold or silver plating is deposited on the aluminum pad. An alloy step may then be used to anneal damage from the argon sputter process and alloy the resultant aluminum to gold or aluminum to silver interface.
In a further embodiment, the aluminum pad layer is made very thin, or less than approximately 8000 xc3x85, to limit Kirkendall voiding caused by gold (Au) atoms from gold wire thermosonic bonds diffusing into the aluminum pad layer.